Compound I species are believed to be the active intermediates in the catalytic cycles of a number of oxidative heme enzymes. Although these reactive complexes are thought to be best formulated as oxoiron(IV) porphyrin-radical cations, recent findings indicate that the axial ligand can have dramatic effects on the electronic structure and suggest that for thiolate-ligated heme proteins an alternative formulation of the compound I species may be more appropriate. Thiolate-heme proteins play critical roles in a number of important physiological processes (e.g. the metabolism of xenobiotics, neurotransmission, blood pressure control, and immune defense against tumor cells). Cytochrome P450 (P450), chloroperoxidase (CPO), and nitric oxide synthase (NOS) all possess an iron protoporphyrin with a cysteinate axial ligand. Recent calculations suggest that the compound I species of these proteins possess sulfur-based radicals. The calculations predict that these sulfur-based radicals result in extremely long Fe-S bond lengths. The formation of the sulfur-based (instead of porphyrin-based) radicals and the resulting long Fe-S bond may be the source of the unique chemistry displayed by these proteins. A detailed knowledge of the active-site geometry and electronic structure is a prerequisite for understanding the chemistry of thiolate-heme proteins. A series of experiments and computations to be performed on thiolate compound I and its one-electron reduced partner, compound II, are proposed. The computations will provide structural and magnetic data for the compound II species and give insight into the dramatic lengthening of the Fe- S bond. The computations will also investigate how hydrogen bonding affects the compound I intermediate. EXAFS and ENDOR experiments will provide evidence of the extremely long Fe-S bond and the large sulfur spin density predicted for the thiolate compound I intermediate. Resonance Raman spectroscopy will be used to investigate the Fe-S and Fe-O bonds.